1. Field of the Invention
This invention relates to a zinc-blende nitride semiconductor free-standing substrate, a method for fabricating the same, and a light-emitting device employing the same.
2. Description of the Related Art
Conventionally, nitride semiconductors applied as blue, green and ultraviolet light-emitting device materials, or high power electronic device materials have wurtzite structure that belongs to hexagonal systems. The nitride semiconductors having wurtzite structure are formed by being grown on a hexagonal substrate, such as sapphire, 4H—SiC, etc. via a low-temperature GaN buffer layer, AlN buffer layer, or high-temperature AlN buffer layer. Using such wurtzite nitride semiconductors, blue violet laser diodes (LDs), blue light-emitting diodes (LEDs), green LEDs, high electron mobility transistors (HEMTs), etc. are put into practical use.
Here, the light emission power of the green LEDs formed from the nitride semiconductors with wurtzite structure is on the order of half the light emission power of the blue LEDs with the same structure. This is because the crystal quality of an active layer of the green LEDs formed from the nitride semiconductors with wurtzite structure is poor compared to the crystal quality of an active layer of the blue LEDs (emitted light wavelength: approximately 460 nm). The reason for the poor crystal quality of the active layer of the green LEDs formed from the nitride semiconductors with wurtzite structure is because the active layer of the green LEDs formed from the nitride semiconductors with wurtzite structure requires that it is formed from an InGaN layer (typically, In composition=0.25) containing more In than the blue LEDs (typically, In composition=0.15), so that the growth temperature of the active layer of the green LEDs formed from the nitride semiconductors with wurtzite structure has to be lower than that of the blue LEDs, and because of large lattice mismatch between an underlying GaN layer and the active layer. Similarly, it is difficult to produce the practical nitride semiconductor green LDs.
As one promising candidate for solving these problems, use of a zinc-blende nitride semiconductor is considered. The band gap energy of a zinc-blende GaN crystal is 3.22 eV, which is lower than the band gap energy (3.42 eV) of a wurtzite GaN crystal by 0.2 eV. Accordingly, by using the zinc-blende nitride semiconductor, enhancement of light emission efficiency of a light-emitting device can be expected. This is because when LEDs or LDs with an InGaN active layer having the same In composition are produced using wurtzite and zinc-blende nitride semiconductors, the zinc-blende structure can emit light with longer wavelength than that of the wurtzite structure because of the band gap energy difference between both.
For example, to obtain 525 nm which is a typical emitted light wavelength of the green LEDs, the In composition is required to be on the order of 0.25 for wurtzite InGaN, but may be not more than 0.2 for zinc-blende InGaN. For this reason, because a high quality InGaN layer can, when using the zinc-blende structure, be formed at higher growth temperatures and with less lattice mismatch as the active layer than when using the wurtzite structure, enhancement of light emission efficiency of the green LEDs can be expected. Further, the possibility of being able to realize the green LDs which was difficult with the wurtzite structure is high.
As a semiconductor light-emitting device using such a zinc-blende nitride semiconductor, for example, a semiconductor light-emitting device is known having a plurality of zinc-blende nitride semiconductor layers formed on a zinc-blende GaAs substrate (see, e.g., JP-A-8-181386). This semiconductor light-emitting device comprises an n-type GaAs substrate, a surface nitride layer formed on the n-type GaAs substrate, an n-type AlGaN layer formed on the surface nitride layer, a GaN/AlGaN multi-quantum well layer formed on the n-type AlGaN layer, a p-type AlGaN cladding layer formed on the GaN/AlGaN multi-quantum well layer, and a p-type GaN contact layer formed on the p-type AlGaN cladding layer. All the layers from the GaAs substrate to the p-type GaN contact layer of this semiconductor light-emitting device are constructed by the zinc-blende crystals.
However, the inventor has found that the conventional zinc-blende nitride semiconductor layers that include the nitride semiconductors forming the semiconductor light-emitting device disclosed by JP-A-8-181386 often include the wurtzite GaN layers at a proportion of not less than a few 10%. When the light-emitting device structure is grown on the GaN layers with a mixture of the zinc-blende and wurtzite structures, the nitride semiconductor active layers grown in the zinc-blende and wurtzite portions emit light with their respective different wavelengths. For this reason, even when producing a green LED, monochromatic green light cannot be emitted, but blue green light is emitted. Also, even when producing a green LD, the monochromatic property of light emitted is poor, so high gain cannot be obtained, and therefore threshold current increases to cause a problem of no laser oscillation at room temperature.